The use of wet scrubbing processes to remove sulfur dioxide from gaseous streams, such as those resulting from power plant combustion systems, has been proposed using calcium scrubbing components such as calcium oxide (lime) or calcium carbonate (limestone). An advantageous such process adds a magnesium scrubbing component and is generally labeled as a magnesium-enhanced scrubbing process.
In calcium-based slurry wet scrubbing processes, inert material is present in the starting lime or limestone that causes problems in later processing, since such inert material is non-reacted in the scrubbing process and usually contains a large amount of silicon, iron and aluminum oxides.
Such inert material is generally crystalline in nature before and after slaking with water to form a slurry for use in the scrubbing process and after introduction into the wet scrubbing system. In such a crystalline form, even after being exposed to slaking reactions and reactions of magnesium and calcium scrubbing components with hydrochloric acid and sulfur dioxide present in a gaseous stream, the solid nature of the inert material enables eventual removal thereof from a sulfur dioxide scrubbing system along with other waste solids without the need for special and costly techniques.
Where magnesium-enhanced calcium scrubbing components are used in wet processes for flue gas desulfurization (FGD), however, and recovery of magnesium hydroxide from the spent scrubbing slurry is made, problems arise with such inert material.
In magnesium-enhanced lime FGD processes similar to that described in U.S. Pat. No. 5,645,807 or U.S. Pat. No. 6,572,832, the contents of both patents being incorporated by reference herein, that employ an oxidation operation separate from the scrubber, the inert material of the original reagent changes to a more amorphous solid structure that becomes extremely difficult to concentrate and purge from the FGD process. Current forced oxidization magnesium-enhanced lime FGD processes have been required to employ complicated and expensive techniques to concentrate, purge and dispose of such inert material.
As the forced oxidized magnesium-enhanced lime FGD process has undergone development beyond the form described in U.S. Pat. No. 6,572,832, it has been found that lime inerts in the amorphous form, caused from exposure to the oxidation step, can be made to revert back to a more crystalline nature if subsequently exposed to a pH of greater than 9.0. The exact mechanism of change, extent of change and rate of change as a function of pH, is yet to be fully determined, however the conditions present in the regeneration tank of the magnesium-enhanced lime FGD byproduct recovery process as described in U.S. Pat. No. 5,084,255 (the contents of which are incorporated by reference, herein), and U.S. Pat. No. 6,572,832 provides an environment where amorphous lime inerts can revert to crystalline structure.
Efforts to remove the amorphous lime inerts (an orange colored fluffy-like material) from magnesium-enhanced lime FGD systems currently involve siphoning the material off from a gypsum fines thickener at an elevation that is above the bed of gypsum fines where amorphous fines are mostly located. The amorphous fines are directed to a separate thickener where a flocculating agent is added to assist settling. At best, such amorphous inerts may settle in the separate thickener to a density of between 5 and 10-wt % before being pumped to centrifuges. More flocculating agent is added to the amorphous inert material slurry that is fed to a centrifuge to assist in further concentrating the amorphous solids to around 20 to 30-wt % density. The resulting concentrated amorphous solid is sloppy and barely able to be handled so that it can be transported to a pug mill where flyash and lime or lime kiln dust is added to create a pozzolonic mixture that can be hauled to landfill with a dump truck. Alternatively a plate and frame pressure filter can be used in place of the centrifuge to concentrate to 40-wt % solids density but the resulting filter cake still needs to be mixed with flyash and lime or lime kiln dust before being landfilled.
Such a concentrating process uses a large quantity of flocculating agent and is labor intensive and expensive.